Britain's military veterans: Why Remembrance Sunday matters so much
Thousands of veterans and civilians joined the King in paying their respects to the fallen in London.
The King was joined by other members of the Royal Family, including the Prince and Princess of Wales, and also the Prime Minister Sir Keir Starmer and Defence Secretary John Healey.
"I can't think of a greater honour and privilege than to be here as Defence Secretary. My message to veterans is simple. Thank you," said Mr Healey.
More than 10,000 veterans from 326 Armed Forces groups were represented by those who marched.
Among those marching was the former head of the British Army, General Sir Patrick Sanders, who is now president of the newly formed Afghanistan Veterans' Community.
He said: "We are here to mark friends who didn't make it back or families who have suffered. It's a bittersweet moment."
Major Ben Clink of the Royal Marines, who took part in the service in London, said: "It's a huge honour for me to be representing the Royal Marines.
"Particularly in our 360th year, and it's a privilege for us to stand on the shoulders of those who came before."
Royal Air Force veteran Henry Paul said: "I was a navigator in the RAF, and I served 34 years. Twenty-five of them at RAF Leuchars.
"We always have a memorial parade here in St Andrews, and we're very lucky to have so many serving military units that can join in."